I recently edited the Wikipedia article 50 Move Rule as follows:
The rationale for this rule is that pawn moves and captures are the only '''irreversible''' types of moves. That is, after each pawn move or capture, it is impossible for any previous position in the game to be repeated.
Unfortunately, my edit was quickly removed with the comment:
This is unreferenced. Come up with a reference if you can.
I think this is pretty clear just by thinking about it, but is there any "formal proof" published anywhere that what I tried to add to the article is true?
EDIT: The words "the only" should be deleted. In addition to captures and pawn moves, losing the right to castle on either or both sides (by moving a rook or king), or failing to capture a certain pawn "en passant" (thus giving up the right to do so later) are also irreversible. These logically should also reset the 50-move count but are omitted from the rules for reasons of simplicity.